Radiation therapy involves medical procedures that selectively expose certain areas of a human body, such as cancerous tumors, to high doses of radiation. The intent of the radiation therapy is to irradiate the targeted biological tissue such that the harmful tissue is destroyed. In certain types of radiotherapy, the irradiation volume can be restricted to the size and shape of the tumor or targeted tissue region to avoid inflicting unnecessary radiation damage to healthy tissue. For example, conformal therapy is a radiotherapy technique that is often employed to optimize dose distribution by conforming the treatment volume more closely to the targeted tumor.
Normal physiological movement represents a limitation in the clinical planning and delivery of conventional radiotherapy and conformal therapy. Normal physiological movement, such as respiration or heart movement, can cause a positional movement of the tumor or tissue region undergoing irradiation. If the radiation beam has been shaped to conform the treatment volume to the exact dimensions of a tumor, then movement of that tumor during treatment could result in the radiation beam not being sufficiently sized or shaped to fully cover the targeted tumoral tissue.
One method to account for the target motion is to simply open up the field aperture to ensure that the radiation volume covers the entire extent of the tumor motion. The problem with this approach is that it irradiates an unnecessarily large volume of healthy tissue.
In another method, physiological gating of the radiation beam during treatment may be performed, with the gating signal synchronized to the movement of the tumor or of a surrogate of the tumor. Such technique may reduce the volume of healthy tissue being exposed to high dose radiation. In this approach, instruments are utilized to measure the physiological state of the patient with reference to the particular physiological movement being examined. For example, respiration has been shown to cause movements in the position of a lung tumor in a patient's body. If radiotherapy is being applied to the lung tumor, then a position sensor can be attached to the patient to measure the patient's respiration cycle. The radiation beam can be gated based upon certain threshold points within the measured respiratory cycle, such that the radiation beam is disengaged during periods in the respiration cycle that correspond to excessive movement of the lung tumor. In some cases, when gating technique is used, the beam may be gated off for a significant amount of time, thereby extending the length of the treatment session.
During treatment, target motion may result in dose intended for a lesion being delivered to normal tissue and organs at risk instead. This requires a strategic treatment decision to mitigate the effect of any anticipated motion in order to preserve the treatment intent. Many methods have been proposed or developed to reduce the dose to normal tissue, including pausing the treatment beam while the lesion is outside the beam (gating), and moving the beam to follow the lesion motion (tracking). It may be difficult, however, to compare the effectiveness of different strategies, and to determine the best strategy for a given patient. In part, this stems from the stochastic nature of the events causing the motion. Furthermore, factors contributing to the relative effectiveness, or ineffectiveness, of each treatment strategy are not always apparent prior to treatment.